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Exploring the Electrical Properties of Twisted Bilayer Graphene Linfield University DigitalCommons@Linfield Senior Theses Student Scholarship & Creative Works 5-2019 Exploring the Electrical Properties of Twisted Bilayer Graphene William Shannon Linfield College Follow this and additional works at: https://digitalcommons.linfield.edu/physstud_theses Part of the Condensed Matter Physics Commons, Energy Systems Commons, Engineering Physics Commons, Materials Science and Engineering Commons, and the Power and Energy Commons Recommended Citation Shannon, William, "Exploring the Electrical Properties of Twisted Bilayer Graphene" (2019). Senior Theses. 45. https://digitalcommons.linfield.edu/physstud_theses/45 This Thesis (Open Access) is protected by copyright and/or related rights. It is brought to you for free via open access, courtesy of DigitalCommons@Linfield, with permission from the rights-holder(s). 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Title of the Thesis: Exploring the Electrical Properties of Twisted Bilayer Graphene Author's Name: (Last name, first name) Shannon, William111 Advisor's Name DigitalCommons@Linfield(DC@L) is our web-based, open access-compliantinstitutional repository for digital content produced by Linfield faculty, students, staff, and their collaborators. It is a permanent archive. By placing your thesis in DC@L,it will be discoverablevia Google Scholarand other search engines. Materialsthat are located in DC@Lare freely accessibleto the world; however, your copyright protects against unauthorized use of the content. Although you have certain rights and privilegeswith your copyright, there are also responsibilities.Please review the following statements and identify that you have read them by signing below. 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My thesis will also be available in print at NicholsonLibrary and can be shared via interlibrary loan. OR I agree to make my thesis available in print at NicholsonLibrary, including accessfor interlibrary loan. OR I agree to make my thesis available in print at NicholsonLibrary only. Updated April 25, 2018 NOTICEOF ORIGINALWORK AND USE OF COPYRIGHT-PROTECTEDMATERIALS: If your work includes images that are not original works by you, you must include permissions from the original content provider or the images will not be included in the repository. If your work includes videos, music, data sets, or other accompanying material that is not original work by you, the same copyright stipulations apply. If your work includes interviews, you must include a statement that you have the permission from the interviewees to make their interviews public. For information about obtaining permissions and sample forms, see https://copyright.columbia.edu/basics/permissions-and-licensing.html. 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Signature Signature redacted Date 5/15/2019 Printed Name WilliamI l D. Shannon Approved by Faculty Advisor Signature redacted Date 5/15/2019 Updated April 25, 2018 Thesis Acceptance Linfield College Thesis Title: Exploring the Electrical Properties of Twisted Bilayer Graphene Submitted by: William Shannon Date Submitted: May, 2019 Research Advisor: Signature redacted Dr. Heath Thesis Advisor: Signature redacted Dr. Crosser Physics Department: Signature redacted Dr. Murray ABSTRACT Exploring the Electrical Properties of Twisted Bilayer Graphene Two-dimensional materials exhibit properties unlike anything else seen in con- ventional substances. Electrons in these materials are confined to move only in the plane. In order to explore the effects of these materials, we have built apparatus and refined procedures with which to create two-dimensional structures. Two-dimensional devices have been made using exfoliated graphene and placed on gold contacts. Their topography has been observed using Atomic Force Microscopy (AFM) confirming samples with monolayer, bilayer, and twisted bilayer structure. Relative work func- tions of each have been measured using Kelvin Probe Force Microscopy (KPFM) showing that twisted bilayer graphene has a surface potential 20 mV higher than that of monolayer graphene and 35 mV below bilayer graphene. For my mother, who supported me in every possible way. Contents 1 Introduction 1 2 Methods 6 2.1 Fabrication . 6 2.2 Sample Selection . 7 2.3 Stamp Creation . 8 2.4 Material Manipulation . 9 2.5 Cleaning Samples . 12 2.6 Data Collection . 13 3 Theory 14 3.1 Moir`ePatterns . 14 3.2 AFM Theory . 17 4 Results and Analysis 19 4.1 Results . 19 4.2 Analysis . 22 5 Conclusion 25 REFERENCES 26 iii List of Figures 1.1 Field Effect Transistor Model . 4 1.2 Cypher Atomic Force Microscope . 5 2.1 Graphene Sample . 8 2.2 Stamp Diagram . 9 2.3 Stamp Process . 10 2.4 Scope Setup . 10 2.5 Creation of Twisted Bilayer Device . 11 2.6 AFM Sample Setup . 13 3.1 AA Versus AB Stacking . 15 3.2 Moir`ePattern . 15 3.3 AFM Diagram . 17 4.1 Twisted Bilayer Data . 20 4.2 Bilayer Data . 21 4.3 Bilayer Regions . 22 4.4 Monolayer Data . 23 iv Chapter 1 Introduction We live in a three-dimensional world. Everything we interact with has length, width, and height. Some things might be thin enough for us to consider them to be essentially \two-dimensional" such as sheets of paper, but an electron living in this paper would consider it to be massively thick. Indeed, making a truly two-dimensional structure in our world is impossible. The building blocks of our world, atoms, have a volume, meaning that everything will necessarily occupy some three-dimensional space. However, that did not stop scientists from trying to get as close as possible. The idea of a material being only one atom thick has been around for quite some time. Since the 1940's people have theorized that it could be possible to create a stable structure with a thickness of only one atom. One of the first of these materials though to be a candidate for being \two-dimensional" was graphite. While you may not recognize the name, graphite is a material that most people have probably used in their life, as it makes up the lead used in pencils. As you write with a pencil, layers of the graphite crystal flake off and remain behind on the paper to leave a visible mark. The crystal structure of graphite and its ability to sheer off into layers made it a prime candidate for being refined down to atomic thickness. The problem was trying to determine how best to actually create the single atom thick lattice. It is not as easy as taking a sharp knife to a piece of graphite and 1 cutting off a slice. The required layer is so thin that no conventional method could be used to create it. In the end, an unexpected tool because the key to creating these two-dimensional materials, Scotch Tape. [1] By placing a flake of graphite on a piece of tape and repeatedly folding it back and forth one can slowly sheer off layer by layer of graphite until you create an atomically flat surface which bonds more strongly to a substrate than its neighboring layer. Indeed, in 2004 Andre Geim and Konstantin Novoselov won the Nobel Prize in physics for the synthesis of graphene using this method. [1] Graphene is a hexagonal honeycomb lattice of carbon atoms only one layer thick. While bulk graphite is a relatively weak material prone to breaking along crystal planes, graphite is the strongest material ever created by man. [2] The electrical properties of graphene are also different than those of graphite. In graphene, electrons are free to move in three-dimensions, while in graphite they are only free to move in two.
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